1. Field of the Invention
The present invention relates in general to detecting systems for detecting the combustion condition of an internal combustion engine, and more particularly to detecting systems of a type which detects abnormal combustion (or misfiring) of the engine by treating an integrated value of the pressure (or combustion pressure) in a cylinder of the engine.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional detecting system of the above-mentioned type will be outlined, which is disclosed in Japanese Utility Model First Provisional Publication 64-15937.
In the conventional system of the publication, the combustion pressure is integrated within a given crankangle range (viz., from TDC (top dead center) to ATDC 30.degree. (viz., 30.degree. after top dead center), and the integrated value is compared with a reference value to judge whether or not abnormal combustion has occurred in a cylinder of the engine. It is said that such detecting system can detect the abnormal combustion more precisely than other conventional detecting systems, such as a system in which the time when the combustion pressure exhibits the maximum value or the increasing rate of the combustion pressure is used as a parameter for detecting the abnormal combustion.
However, even the detecting system disclosed by the publication has failed to exhibit a satisfied detecting ability particularly in a case wherein the engine is under a low load and low rotation speed condition. That is, under such condition, the combustion in each cylinder is unstable, and the combustion pressure tends to exhibit a marked dispersion. Furthermore, the integrated value of the combustion pressure derived under such condition fails to show a marked difference from that derived under normal combustion, and thus, it is difficult to keep the high detecting ability throughout substantially whole operation range of the engine.